


Oathbound

by 0neType



Category: Undertale (Video Game)
Genre: Biting, Hypnosis, Implied Rape/Non-con, M/M, Magic Drinking, Pre-Canon, Vampire Gaster AU
Language: English
Status: Completed
Published: 2016-09-11
Updated: 2016-09-11
Packaged: 2018-08-14 08:46:45
Rating: Mature
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 1
Words: 4,489
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/8006245
Author URL: https://archiveofourown.org/users/0neType/pseuds/0neType
Summary: <blockquote class="userstuff">
              <p>"At that point, Sans had smiled at him, slow and self-assured. He had the look of a monster with all the right cards in hand, ready for anything the world could throw at him. Gaster felt his soul stutter still in his chest."</p><p>Gaster is more terrible than he believes he is. Sans, unfortunately, has no idea until it's too late.</p>
            </blockquote>





	Oathbound

**Author's Note:**

  * For [sweetsinnerchild](https://archiveofourown.org/users/sweetsinnerchild/gifts).



> ~~ily bro <33 //smooches~~
> 
> important note!! there's implied non-con in this fic but it's nothing graphic so i didn't use the archive warning. please be cautious in proceeding if that sort of thing messes with you <3

There’s a new monster on Gaster’s team.

This, actually, is fairly newsworthy all on its own.

Gaster is the Royal Scientist after all—power, prestige and a lifetime of credentials to back up his position—and to be working directly under him means being the guaranteed best of what the Underground has to offer.

There are no wanted ads, no job listings on the UnderNet, not even any whispers through well-connected acquaintances that encourage people to apply for a place by his side. Gaster offers his tutelage in person, as thorough in his hiring as he is with everything else. A monster approached by him is one that has been lucky enough to be able to catch his interest. A place on his team is nothing short of a blessing and certainly not a gift to take lightly.

Which makes the new scientist even more of an anomaly.

Sans had turned up at the doors to his lab unannounced, refusing to leave till he met with the Royal Scientist.

Such a thing should’ve been unthinkable.

The entire area was heavily guarded; well-trained monsters, cameras and the latest tech developed by sub-divisions working under him. Gaster had given a rare frown of heavy disapproval when the matter had been brought to his attention by a nervous looking guard. He was a busy man—matters of security should be dealt with without having to concern him.

However, he had also always been a curious man by nature, and his need to discover how the tiny monster on the security feeds had managed such a feat won out over his instinctive urge to alert the authorities.

He’d called for the monster to be brought straight to his main office.

When Sans had entered, Gaster expected smugness; a prideful monster with an ego to boot. It would have been understandable, considering his cunning getting this far. What else could he have expected, he reasoned, when the tiny thing had demanded to be met despite clearly trespassing on restricted grounds. His surprise was difficult to mask when what he’d got instead was a dishevelled looking monster, clear cut confidence apparent in his stance but a spark of desperation in his eyes that he was not quick enough to hide from the scientist’s discerning gaze.

He’d been intrigued.

“What are you after?” Gaster had asked, straight to the point.

Sans hadn’t looked startled by his blunt opening at all. He’d simply shrugged off the hold of the guard walking him in and stood in front of his desk, fists clenched at his sides and posture straight. His eyelights had glinted fiercely as he matched Gaster’s gaze.

“A job.” He’d said, voice clear and composed.

He’d looked the small monster over then, pointedly lingering on his rumpled appearance. Dirty, torn clothes and mismatched socks worn under too-big shoes. His shoelaces were tied, surprisingly enough, but that didn’t distract from the overall unkempt impression he gave off.

Sans was a mess.

Homeless most likely.

Gaster had leaned back in his plush leather chair, fingers laced together atop his polished desk, “What makes you think you’d be able to contribute anything worthwhile to my research?”

Sans had taken a lengthy pause then, stared up at him with steely conviction. It was an engaging expression if one could be bothered to look past all that grime. Gaster had found himself unconsciously shifting closer to see it better.

Sans had kept his voice level, “I can help you with the Core.”

 _That_ had gotten to him.

Gaster had nearly shot right out of his seat at that, immediately gone into high alert. It had taken him a moment to register that there hadn’t been any hint of malice in Sans’ tone. He’d forced himself calm, gripped his enlaced hands tighter.

“That,” He’d started, watching Sans closely for any signs of trouble, “Is classified information. How in the stars did you even hear its name?”

At that point, Sans had smiled at him, slow and self-assured. He had the look of a monster with all the right cards in hand, ready for anything the world could throw at him. Gaster felt his soul stutter still in his chest.

It hadn’t mattered what Sans said afterwards.

Despite himself,

Gaster had already been sold.

 

\---

 

Sans is nothing like the other monsters on his team.

He’s just as dedicated, sure—he’s always on top of his work, never a calculation done out of place. He’s not shy with his ideas either, ready and willing to speak up when he’s thought of something new. All in all, a perfect fit for his division.

What’s _different_ is his behaviour in the moments in between.

For one thing, never before has Gaster had to deal with an employee snickering so often.

In fact, the first time Gaster hears Sans’ laugh, he doesn’t even recognise it for what it is. It starts after a rare stutter in his speech as his mouth tries to keep pace with the information pushing rapidly through his head. He stops right in the middle of his lengthy lecture on new procedures, darts a confused glance around the room for the source of the noise. It takes him till Sans starts laughing harder at the confusion on his face before he even looks his way.

There’s easy mirth in his expression, eyelights sparkling with the kind of joy Gaster hadn’t thought he’d be capable of.

He’s read the file—had a background check done on Sans immediately after hiring him—he knows about his life and about all his difficulties. He knows Sans has a brother to raise and not a penny to his name. He knows exactly the sorts of sacrifices that Sans has made in order to get here today.

Even for someone like Gaster, who has gone through more than a lifetime of sorrow, it is a harrowing tale.

So where does this unhurried amusement come from?

Where do all these bright grins and soft smiles find time to form?

Surely there must be something to it? Something more than just simple joy at ridiculous antics about his humdrum workplace. Gaster could understand maybe, if it was something deeper motivating it.

But there’s no secret to be found.

Sans simply _is_ ; bright and compassionate and earnest in all that he does.

It’s…

… perplexing.

But Gaster can’t say he hates it.

 

\---

 

“Ahh—”

There’s another reason Gaster handpicks those that work for him.

He needs very specific kinds of monsters. Nothing that requires a checklist, more just two specific traits that he keeps aware of at all times. He wants those that are smart, certainly, but he’s of a mind that anyone can do the work required of a genius if guided by the right hand.

What’s more important is that they’re strong of magic.

Strong of magic and weak of will.

“Shh…” Gaster soothes, dragging his tongue along the warm expanse of skin at his teeth, “I will be quick.”

The monster in his arms is limp and unresisting, eyes glazed over with the haze of total obedience. They will not remember this afterwards, Gaster is sure in this. He’s done this exact act enough that there are no mistakes. No slip-ups and no miscalculations.

There never are and never will be any trails to lead back to him.

He sinks his teeth into the tender flesh at his disposal and the monster gripped in his arms makes no protest. His magic blankets them, keeping all other thought at bay. They stay relaxed and yielding in his grasp.

This is necessary, he reminds himself.

Gaster is the last of his kind; a species of monster that hasn’t existed since the years up on the surface. His race had been wiped out forever ago, in battles and massacres of decades’ past. He’d lost much to those wars, and had further forfeited ever knowing peace when he’d made his decision to follow the fleeing monsters underground. He’d reasoned it was better than dying under the sky he was born beneath, fighting a one-sided fight.

At least this way he could contribute something worthwhile one day; something that could perhaps change the face of every battle they fought from then onwards.

But he doesn’t deny that he sometimes misses the ease of the old days. Where, once upon a time, there was no need to resort to all this.

His subspecies of monster sustained themselves purely on the magic of others of their kind. It was a social event; a symbiotic sort of thing that gave as much as it took. Children would feed on their parents as they grew, their parents in turn feeding on them in small amounts. Spouses and lovers would feed on each other during quiet moments or throes of passion. Friends, colleagues and even acquaintances getting to know each other for the first time would engage in the act.

It was easy; simple, natural, and never something to hide.

When Gaster had first made his way Underground, in the midst of his mourning he’d be struck with the fearful realisation that he’d never feed again. That he’d slowly, agonisingly waste away till he was dust, starving surrounded by food he could not consume to any benefit. It was a waste, he’d thought, to have come down at all; dust was of no help to anyone.

But he’d always been a scientist at heart, even then when he’d just been climbing up and making a name for himself. And in a dark moment of contemplation he’d wondered, just what would happen if he fed on a monster that was not of his own?

So he’d experimented.

And he’d learned.

It was not as simple as it was with others of his predilection—for one, monsters he drank from were left dazed and drained in a way that had never been an issue before. It had seemed that since there was no way to offer his magic in return, he now played the part of a parasite, taking what magic he could without killing them and giving nothing to soothe the absence in return. The monsters were left weak and wobbly, time being the only remedy to let their bodies heal enough to replace that which had been stolen from them.

For another, it was incredibly different to find anyone willing to offer themselves to him in this manner. It seemed it goes against the nature, and indeed the survival instincts, in most monsters to readily give up even a portion of that which sustained them. He found, in his darker moments, that most days it was easier to simply take what he wanted instead, using a gentle persuasion of the minds of the weak that came to him as natural to him as breathing.

It was not ideal but it was the only way to survive.

… it was also the first of many secrets to keep.

“Hah…” The monster in his grasp exhales softly as his fangs penetrate their skin. His mouth closes over the magic that’s drawn from their body, live-giving liquid filling in against his tongue. There is no time to savour the taste; there never is.

Gaster drinks.

A sudden crash rings in the air.

The violence of the sound has Gaster ripping away from the monster in surprise, teeth tearing at the puncture wounds he’d made and causing a muted cry to come from the creature still spellbound by his magic. He looks up, soul thudding hard in his chest. He meets a wide, familiar gaze, eyelights pinpricked and flickering.

Sans stares at him, books and papers scattered on the floor by his feet.

Gaster doesn’t know what to say.

He can’t remember the last time he got caught.

“Sans…” He starts but does not continue.

The silence lingers between them.

“I-I didn’t see anything,” Sans says after making a visible effort to tear his gaze away from the scene in front of him. His voice is remarkably put together considering how plainly his eyes speak of the horror he feels; hardly shakes at all. It’s almost admirable, “I won’t tell a soul. I swear.”

Gaster stares at him, quiet and contemplative.

He wipes the magic from his mouth.

 

\---

 

Sans needs this job. That much has been clear since the moment he stepped through Gaster’s door.

Perhaps this is the reason why nothing changes.

Sans is true to his word and there is no sudden spill of officials pouring into his labs to demand he turn himself in. Instead, the days progress as normal, his team working tirelessly away to create something strong enough to power the entire Underground. No one seems any more aware of Gaster’s late night indulgences than they usually are.

The short skeleton himself is also unchanged.

He continues to joke and laugh, nudging at his lab partner with a smile when they groan good-naturedly at his antics. He’s still open, still honest despite what he now has to hide. He doesn’t try to use the knowledge he has as a bargaining chip either. Does not slack or let any of his work suffer with the notion that Gaster will keep him on as long as there’s something to blackmail him with. All Sans’ tasks are up-to-date. Every decimal is kept in line.

Sans does not waver in the slightest.

However, most surprising perhaps, is that even with Gaster he still acts freely.

“You are not in appropriate lab attire, Sans.” He lightly scolds one morning, eyeing the ensemble Sans has on under his lab coat, “Please take care to dress in accordance with protocol in the future.”

“Aww, c’mon, Doc. No need to be so _shorts_ with me.” Sans grins, shaking an uncovered leg at him, friendly and teasing.

Gaster stares at him for a moment before turning away, not a word spoken. He feels his soul beat quickly in his chest and he rubs surreptitiously at it, slow and unpracticed. His whole body feels a little warm.

 

\---

 

It is weeks after the event that he invites Sans back to his office.

“You wanted to see me, Doc?” Sans says as he shuffles in past the door.

Gaster can plainly see that he’s nervous. His posture is shrunken, making his already small form seem even tinier. He has his hand stuffed into the pockets of his lab-coat, stiff and forced casual. The scientist has a feeling that it’s likely out of a need to keep them from shaking out in the open. He ignores the way his soul pains at the thought.

He tries to keep his tone unthreatening, “I believe we have something to discuss.”

He readies himself for a joke, for a chuckle and a couple of words thrown out to distract from the matter at hand. He prepares to cast them all off. He needs Sans to be serious here. He needs to know that the young monster before him fully understands the weight of the secret Gaster needs him to keep. He’s never shared the burden of his condition with another before, never even felt the inclination to do so. He’s not sure he knows how.

But, for some reason, he now finds himself willing to try.

As it turns out though, he doesn’t need the preparation.

Sans nods at him, short and stilted, “I’m ready.”

Gaster stares at him, uncomprehending.

“There’s just one thing though,” Sans drags his gaze up at him then and Gaster is struck with the image of him standing in the same place so many months ago, in his awful, awful clothes, wearing the same resolute expression, “I don’t want you to put me under.”

He continues to stare, blinking slowly at the monster in front of him, “What?”

“That thing you did the other day. To Romy.” Sans begins to looks frustrated when Gaster does not follow up with a response, frowning deeply, “Look, I asked them about it later—no specifics, just why they’d gone to your office in the first place—but they had no idea what I was talking about. Didn’t remember a thing.”

“Oh. Yes. That is…” Gaster finds that it’s difficult to look Sans in the eye as he speaks, especially when his mind is whirling with the possible outcomes to what Sans is saying, “That is necessary.”

“I don’t care.” Sans is firm, gaze fixed and hardened, “I don’t like the idea of losing myself like that. Of not knowing what I did.”

There’s a frantic pounding in his soul that Gaster hasn’t had cause to feel in _years_ because—Sans is under _completely_ the wrong impression. He’s misconstrued exactly why Gaster has brought him into the office, has built up his own idea of what’s expected of him. He seems to think he’s under obligation to let Gaster feed from him as part of his job, that he’s only been given a reprieve because of what he’s seen.

Gaster doesn’t know if Sans believes he’s done this already and just doesn’t remember or not.

It doesn’t matter.

What’s important is that Sans is offering himself up to him.

Sans is _offering_.

“I see.” He says, short and succinct even as a bubble of excitement grows inside of him.

Sans is naïve; he has no idea what he’s talking about.

What he does when he feeds is nothing so simple as a memory wipe. It is coercion at its most primal. Basic instincts warped and shaped to his own benefit. The fact that he blesses his victims with a blank memory of the event is entirely separate from the magic it takes to bend their wills far enough to allow it in the first place.

But it’s been necessary. Unavoidable.

Because no one actually ever _offers_.

“I will not make you forget.” He promises.

 

\---

 

With Sans, Gaster is able to savour the taste of the magic that floods into his mouth like he hasn’t had the pleasure of since being sealed behind the barrier.

It’s warm in his mouth, a heady mixture of sweet and tart that he laps at eagerly. It’s pinched with a metallic tinge from the marrow that seeps in alongside it but that only adds to the burst of flavour that rocks against him with every pulse of Sans’ soul. Even the texture of it is amazing, thick and smooth at his tongue, slicking his mouth as he drinks.

He loves it.

He loves the taste and he loves the way Sans clings to the front of his lab-coat as Gaster pulls more of the magic from him.

He loves the small, half-bitten sounds the skeleton makes as he struggles to contain his reactions, face flushing warm from their closeness and the easy hold Gaster keeps on him.

He loves it all.

 

\---

 

He does not make Sans forget.

He keeps his word and things are better for it.

It’s freeing being able to come into work the morning after another session with Sans and to see him smiling at him just the same as always. There’s no secrecy, no distance. Only an easy comradery that Gaster has never before had a chance at.

Sans is his usual self, as he always has been since the day he first arrived. He is steadfast and hardworking when the situation calls for it and playfully lazy during their off-time. He’s a nuisance with his pranking but beloved by his lab partners anyway, easy conversation flowing in the cafeteria over food and drink. He still does not shy away from Gaster, prodding at him for answers and brushing easily by his side while looking for equipment. Things are good.

Better than they’ve been in a good long while.

If the others working for him notice his increased closeness with Sans, they do not mention it.

 

\---

 

When Gaster drinks from Sans this time, he lets his hands wander.

The skeleton is sitting in his lap, the back of his spine pressed against Gaster’s chest as he drinks steadily from the gentle slope of his neck. His fangs always sink so easily into the bones there, as if they were made to be taken by him, soft and pliable under his touch. He pulls back and licks at the punctures he leaves, moves slightly further up along Sans’ vertebrae to pierce him again.

Sans gives a hiss as his teeth sink in once more and Gaster lets his hands move from where they rest at Sans’s femurs to move up and grab firmly at his hips.

Sans’ breath catches.

 

\---

 

There are some changes.

For one, Sans has bought new clothes.

Apparently he has now saved up enough disposable income to do such a thing and Gaster is treated to the sight of Sans dressed up far better than he usually is. He’s also seemed to taken up a preference for turtlenecks, and it sends a spark of something warm and almost possessive in Gaster’s chest to see that. The high fabric brushes up against the bottom of his chin, covering up inches of pale, ivory bone. He knows exactly what Sans is trying to hide beneath the high collars.

The second change is far less enjoyable.

Sans has gotten a lot more antsy lately, a twitching bundle of nerves that only seems to become readily apparent when he notices Gaster in the room. It’s kind of charming at first, the way Sans jumps up in surprise as Gaster comes up behind him or the sudden meekness in his demeanor if he makes eye contact for too long but Gaster quickly grows tired of it. He likes Sans best the way he always has been.

Besides, there’s no reason for the monster to be so shy around him anyways.

 

\---

 

“S-stop.” Sans stutters against him and Gaster freezes with his hands working their way underneath his sweater.

He pulls his head back, detaches himself from Sans’ collarbone and takes a second to admire the numerous bite marks littered across the once smooth expanse of bone. He lets his hands linger where they are and directs a questioning glance at the monster straddling his lap. Sans doesn’t see it, eyes turned down.

So Gaster speaks instead, “What is it, Sans?”

“D-don’t,” Sans forces out, an uncharacteristic hesitance to his words, “Don’t touch me.”

Gaster returns his hands to Sans’ hips, grips them firmly, frown overtaking his face, “You _offered_ , Sans.”

The skeleton shakes his head wildly, arms trembling where Gaster had placed them atop his shoulders, “N-no. Not—not _this_. I didn’t offer _this_.”

The scientist stares at him, slightly annoyed by his behaviour. He’s uncertain if this is the small monster’s attempt at playing coy but it’s disingenuous and Gaster doesn’t much care for it. After all, if Sans truly had a problem with a few harmless touches, why wouldn’t he have said anything earlier?

Nevertheless, he lets his grip loosen, and takes note of the relieved sigh that escapes from Sans as he does. He files it away and returns to the more important task at hand. He drifts back to Sans’ neck, eyes the lovely blue-black bruising of bone where he’s pierced him again and again.

He sinks in against him and Sans yelps, phalanges gripping tight into his shoulder.

He drinks deeply.

 

\---

 

He loses himself in Sans’ taste.

It’s been weeks now and yet, somehow, Sans tastes just as good as the first time. Potent and strong, magic in him overflowing with vitality. Sans’ flavour is uniquely compelling, enough to make Gaster moan, lapping at the dregs of magic that spill as he pulls away to re-enter him elsewhere.

In his high, he presses his magic smeared mouth against Sans’. The skeleton parts his teeth with a startled gasp and Gaster uses the opportunity to coax his tongue in, tasting Sans’ anew. It’s exhilarating. Extraordinary even as Sans starts to struggle in his grip. He brings a hand up to press against the back of Sans’ skull, keeps him in place as the scientist explores his mouth.

This time, when Sans protests, pushing urgently against his chest, he instinctively pushes back with a layer of his own magic.

Sans immediately goes still and docile in his grip.

Gaster stares at him.

It’s easier than he expected.

 

\---

 

He has acquired a taste for Sans.

Drinking from anyone else leaves a bitter taste in his mouth, unsatisfying and lackluster.

He’s been calling on Sans more and more often these days, forgoing a meal from anyone else. It’s just not the same he insists to himself. It’s better when the monster he’s drinking from is complicit in what’s happening.

It’s better when he’s not the only one who remembers what’s taken place.

The attention from him is making Sans a little weary, he knows. Others in the lab have started noticing it too, asking him worried questions in hushed voices. They ask after his health, after his brother’s health—and it’s funny, Gaster had entirely forgotten that Sans had family—about how much sleep he’s getting. It would be enough to make Gaster feel bad if it wasn’t for the warmth that sprang from his soul at even just the thought taking Sans once more.

In any case, the monster in question always shrugs it all off with his usual carefree attitude so it’s not like it really matters. And if there’s a haggard undertone to his words and the tightness of a lie at his teeth, no one but Gaster really notices anyway. Sans just needs a little time to adjust, is all.

“Meet me in my office afterwards, Sans.” He says as he leaves a lingering touch to the skeleton’s shoulder.

Sans studiously avoids looking at him.

 

\---

 

Sans is always complacent till Gaster comes; always moaning and imploring and wholly enthusiastic till the few seconds of his climax where he loses his hold enough for Sans to react.

At first, he had used those moments to struggle, to put up a fight. Biting and kicking and screaming. Trying to put the magic Gaster hadn’t yet drained to force him off. But the scientist had firmly put an end to that; had been glad that Sans’ brother was useful for leverage if for nothing else.

Now, Sans only cries in the aftermath.

He whimpers like a wounded animal as he lays naked against the coarse wood of Gaster’s desk. Evidence of Gaster’s release glistens against his bones between the torn punctures where the scientist had sunk his teeth into him. Gaster feels his breath catch. Sans is so beautiful like this. Just looking at him spread open in this manner, thoroughly debauched, makes the heat rise back up inside of him.

“Please,” Sans begs, voice hoarse and broken, “Please make it go away. I don’t want this anymore. I don’t want to _know_.”

“I made you a promise, Sans,” Gaster whispers as he leans down over the frail body underneath him. He brushes a gentle hand against Sans’ cheekbone, kisses his tear-streaked face, “I will not break it.”


End file.
